With the development of integrated circuits (IC), a diameter of the wafer can reach more than 300 mm and a characteristic line width of the wafer can reach less than 45 nm. Conventionally, new semiconductors, conductors, and dielectric materials are applied to overcome problems caused by the increase of integration degree, such as high power consumption and signal delay. A rapid development of IC is limited by copper interconnection. Accordingly, mediums with lower k, smaller line width, and multiple layers are effective methods to make an improvement. However, a medium layer with low k has a mechanical strength much smaller than copper lines, so that the media layer may be easily damaged during a high-stress CMP (chemical mechanical polishing) process. In order to solve the above problems, a two-step polishing method is applied. The copper layer may be firstly polished to a certain extend with the high-stress CMP (the first step), and secondly polished without the use of stress (the second step). Before the second step is performed, the thickness of the copper film on each desired points of the workpiece needs to be measured. An eddy current method is a non-contact measuring method, which does not damage the copper film during the measuring process and has high accuracy. However, in a conventional measuring device, the lift-off (i.e. a distance between a measuring sensor of the measuring device and the film to be measured) is not stable, which has a bad influence on the measuring accuracy.